1. Field of the Invention
This invention relates generally to tracking control apparatus, and more particularly is directed to an improved tracking control apparatus suitable for use with a digital signal reproducing apparatus.
2. Description of the Prior Art
The assignee of the present application has previously proposed a method for controlling tracking of a rotary reproducing head and which uses only a reproduced output from such rotary reproducing head, that is, which effects tracking control without the need for a stationary head.
This tracking control method is based on the idea that a PCM (pulse-code-modulated) signal can be easily timebase-compressed and/or timebase-decompressed and, therefore, the PCM signal is not necessarily recorded and/or reproduced continuously or in real time as is required in the case of an analog signal. Thus, the PCM signal can be recorded in one area of each of a succession of tracks on the record medium and other signals, can be recorded in other areas which are different from the recording area for the PCM signal in each track.
Specifically, when the PCM signal is timebase-compressed and recorded on a record medium by a plurality of rotary heads so as to form successive slant tracks without any guard bands therebetween, pilot signals for use in tracking are recorded on an area of each track which is independent, in the longitudinal direction of the track, from the recording area for the PCM signal. Upon reproducing, each track is traced by a rotary head whose tracing width is wider than the width of the track and the tracking alignment of the rotary head is controlled with reference to reproduced pilot signals derived from both tracks adjacent to the track which is being traced by the rotary head.
In a tracking alignment control method of the above type previously proposed by the assignee of the present application, a plurality of position detecting signals which differ in frequency for adjacent tracks and which are different in recording length for adjacent tracks having the same frequency, are recorded in the tracks in accordance with a repeating sequence of a predetermined number of record tracks.
Upon reproducing, when a record track is traced by a rotary head whose tracing width is wider than that of the track, a pulse signal is formed on the basis of the starting point of a position detecting signal recorded in such track and, at the time of this pulse signal and in response thereto, a pilot signal is detected from a track adjacent to the track being traced by the rotary head and tracking alignment of the rotary head is controlled with reference to the detected pilot signal. This previously proposed method is disclosed in detail in U.S. patent application Ser. No. 761,025, filed July 31, 1985, and having a common assignee herewith. Moreover, said U.S. application corresponds to EP Laid-Open patent application Ser. No. 0171266. In such disclosed method, during reproducing, when an identifying signal is at a low level, a position detecting signal having a long recording length is detected, and, when the identifying signal is at a high level, a position detecting signal having a short recording length is detected. If the identifying signal results from frequency-dividing a switching pulse or change-over signal, a sampling pulse used to detect the tracking error is generated only when the recording length of a position detecting signal detected from the track being more or less traced by the rotary head coincides with the polarity or level of the identifying signal for detecting a position detecting signal of that recording length.
However, when the signal for identifying tracks having position detecting signals with different recording lengths is generated independently of the signal reproduced from the record medium by the rotary head, as is the signal resulting from frequency-dividing the switching pulse in the above-mentioned U.S. patent application Ser. No. 761,025, there occurs the following problem. Even in the on-track state, that is, when there is no tracking error, if the identifying signal is not coincident in polarity or level with the recording length of the position detecting signals in the track being traced by the rotary head, no sampling pulse for detecting a tracking error is generated. As a result, upon reproducing, if the rotary head is once diverted from the on-track state, for example, due to the drift of the tape speed, the rotary head becomes stable after the rotary head traces the track once removed from the adjacent track. In other words, the tracking is not stablized until the rotary head traces the track once removed from the adjacent track, thus increasing the time required for the tracking control to be carried out in the stable manner, that is, the pull-in time.